Patients with pathologically proven NPC, stage II-IVB and finishing curative RT ≧70 Gy within 6 months (± induction/concurrent/adjuvant chemotherapy) will be candidates for this study. Before entry, 8 CC venous blood will be obtained for EBV DNA and antibody screen tests after patient's consent. Those who have undetectable plasma EBV DNA (0 copy/ml) and fulfilled with all inclusion and exclusion criteria will be registered.